Fix randomly failing test TestThumbnailStorage.testCreation

Assert.assertNotNull(sameFile.getPropertyValue(ThumbnailConstants.THUMBNAIL_PROPERTY_NAME));

java.lang.AssertionError: 
	at org.junit.Assert.fail(Assert.java:91)
	at org.junit.Assert.assertTrue(Assert.java:43)
	at org.junit.Assert.assertNotNull(Assert.java:524)
	at org.junit.Assert.assertNotNull(Assert.java:535)
	at org.nuxeo.ecm.platform.thumbnail.test.TestThumbnailStorage.testCreation(TestThumbnailStorage.java:92)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)
	at org.nuxeo.runtime.test.runner.FeaturesRunner$InvokeMethod.evaluate(FeaturesRunner.java:342)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:76)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
	at org.nuxeo.runtime.test.runner.FeaturesRunner.run(FeaturesRunner.java:289)
	at org.apache.maven.surefire.junit4.JUnit4TestSet.execute(JUnit4TestSet.java:53)
	at org.apache.maven.surefire.junit4.JUnit4Provider.executeTestSet(JUnit4Provider.java:123)
	at org.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:104)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at org.apache.maven.surefire.util.ReflectionUtils.invokeMethodWithArray(ReflectionUtils.java:164)
	at org.apache.maven.surefire.booter.ProviderFactory$ProviderProxy.invoke(ProviderFactory.java:110)
	at org.apache.maven.surefire.booter.SurefireStarter.invokeProvider(SurefireStarter.java:175)
	at org.apache.maven.surefire.booter.SurefireStarter.runSuitesInProcessWhenForked(SurefireStarter.java:107)
	at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:68)